Update dragDevices doc to include default PointerDeviceKind.trackpad (#163898)

[ScrollBehavior.dragDevices](https://api.flutter.dev/flutter/widgets/ScrollBehavior/dragDevices.html)
docs say

> By default only PointerDeviceKind.touch, PointerDeviceKind.stylus, and
PointerDeviceKind.invertedStylus are configured to create drag gestures.

However, `PointerDeviceKind.trackpad` is also included by default
flutter/flutter#89944


https://github.com/flutter/flutter/blob/401ab831ee56bb31b8beceb0a08bc3293f1a9788/packages/flutter/lib/src/widgets/scroll_configuration.dart#L115-L119


https://github.com/flutter/flutter/blob/401ab831ee56bb31b8beceb0a08bc3293f1a9788/packages/flutter/lib/src/widgets/scroll_configuration.dart#L33

Update the comment to include `trackpad`:
> By default only PointerDeviceKind.touch, PointerDeviceKind.stylus,
PointerDeviceKind.invertedStylus, **and PointerDeviceKind.trackpad** are
configured to create drag gestures.



## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

Author: jmagman

packages/flutter/lib/src/widgets/scroll_configuration.dart

@@ -112,10 +112,11 @@ class ScrollBehavior {
 
   /// The device kinds that the scrollable will accept drag gestures from.
   ///
-  /// By default only [PointerDeviceKind.touch], [PointerDeviceKind.stylus], and
-  /// [PointerDeviceKind.invertedStylus] are configured to create drag gestures.
-  /// Enabling this for [PointerDeviceKind.mouse] will make it difficult or
-  /// impossible to select text in scrollable containers and is not recommended.
+  /// By default only [PointerDeviceKind.touch], [PointerDeviceKind.stylus],
+  /// [PointerDeviceKind.invertedStylus], and [PointerDeviceKind.trackpad]
+  /// are configured to create drag gestures. Enabling this for
+  /// [PointerDeviceKind.mouse] will make it difficult or impossible to select
+  /// text in scrollable containers and is not recommended.
   Set<PointerDeviceKind> get dragDevices => _kTouchLikeDeviceTypes;
 
   /// {@macro flutter.gestures.monodrag.DragGestureRecognizer.multitouchDragStrategy}